This invention relates generally to thin film transistor (TFT) processes and fabrication, and more particularly, to a polycrystalline film, and method of forming the polycrystalline film, from a microcrystalline film.
Low-temperature polysilicon formation is a process that has been intensively investigated for more than a decade now. These investigations have become more important due to the potential of this material in flat panel display related technology, particularly, in the area of active matrix liquid crystal displays (AMLCDs). Polysilicon TFTs offer the advantages of: (a) smaller device dimensions, thus higher aperture ratio for the display; (b) higher TFT "on" current, thus less sensitivity to gate and bus line signal delays; and, (c) elimination of external drivers and interconnects, thus possible yield and cost improvement (especially in small size displays).
Earlier efforts focused on the formation of polysilicon by SPC methods (i.e. low temperature furnace anneal). Recently, significant emphasis has been placed on excimer laser anneal as a more suitable process for the formation of high quality polysilicon material from an amorphous-silicon precursor film. ELA technology has significantly matured over the past decade. A number of commercial tools are currently available to flat panel display manufacturers. However, problems still plague this process. Control of grain size is difficult, especially under operation close to the super-lateral-growth (SLG) regime, where small variations in laser energy density result in large variations in grain size. Uniformity of grain size is another major issue, especially in the overlap region between successive laser beam shots. Moreover, damage of the underlying substrate is another problem, typically encountered as the energy density of the laser increases to accommodate a larger grain size in the annealed polysilicon layer.
In light of these problems, it is desirable to develop a process that can compensate for some of the intrinsic shortcomings of ELA. More specifically, it is desired to obtain high quality polysilicon at the minimum laser energy "load", to avoid hardware instabilities and substrate damage. This issue becomes even more important as technology moves to cheaper substrates, such as soda-lime glasses or organic substrates. Furthermore, it is desirable to reduce the variability of the polysilicon material characteristics by appropriate engineering of the structure of the silicon film precursor. The method of depositing amorphous silicon on the transparent substrate is also crucial in the fabrication of polycrystalline films having large crystal grains. Previous work in the area of solid phase crystallization of silicon has indicated that certain control of the structural characteristics of polysilicon is possible through materials engineering of the phase of the as-deposited film. Recent evidence has found that the starting phase of the film affects the crystalline characteristics of the post-ELA (excimer laser annealed) polysilicon.
Typically, an LCD is made by mounting a transparent substrate on a heated susceptor. The transparent substrate is exposed to gases which include elements of silicon and hydrogen. The gases decompose to leave solid phased silicon on the substrate. In a plasma-enhanced chemical vapor deposition (PECVD) system, the decomposition of source gases is assisted with the use of radio frequency (RF) energy. A low-pressure (LPCVD), or ultra-high vacuum (UHV-CVD), system pyrolytically decomposes the source gases at low pressures. In a photo-CVD system the decomposition of source gases is assisted with photon energy. In a high-density plasma CVD system high-density plasma sources, such as inductively coupled plasma and helicon sources are used. In a hot wire CVD system the production of activated hydrogen atoms leads to the decomposition of the source gases.
It would be advantageous if a high quality polycrystalline film could be obtained by excimer laser crystallization, at low levels of laser energy density.
It would be advantageous if crystallization process steps could be reduced by optimization of the deposition and annealing procedures. It would also be advantageous if good crystal structural characteristics could be obtained with a low variability against disturbance factors (i.e. variation in laser energy density and/or other process parameters).
It would be advantageous if a polycrystalline film could be fabricated on a transparent substrate having an electron mobility of 150 cm.sup.2 /Vs, or more.
Accordingly, a method of forming a polycrystalline film having high electron mobility and low threshold voltage is provided. The method comprises the steps of:
a) depositing a microcrystallite film having a microcrystallite density and a microcrystallite size; PA1 b) annealing the microcrystallite film; and PA1 c) in response to the microcrystallite density and size and the annealing, forming a polycrystalline film having a polycrystalline grain size and a polycrystalline grain size uniformity. The embedded microcrystallite seed crystals surviving annealment form nucleation sites in the polycrystalline film. That is, the crystal grains are directly responsive to quantifiable variables such as microcrystallite size, density, and annealing energies.
Specifically, the step of annealing involving heating the microcrystalline film, melting the amorphous matter, selectively melting microcrystallites, and cooling the microcrystalline film. A second density of unmelted microcrystallites are left embedded in the molten amorphous matter. Then, Step c) includes crystallizing the amorphous matter melted in Step b), using the unmelted microcrystallites as nucleation sites. The polycrystalline film grain size is responsive to the size and density of unmelted microcrystallites.
Generally, the microcrystalline is formed on a first region adjacent, and overlying the transparent substrate. Step b) includes selectively melting the microcrystalline film so that the second density of microcrystallites is primarily in the first region adjacent the transparent substrate.
Step a) includes forming a first microcrystallite crystalline fraction which is a product of the first microcrystallite size and the first microcrystallite density, and in which the first crystalline fraction in the range of approximately 0.01 to 80%. In some aspects of the invention, where larger sized microcrystallites are used, the pre-melt (as-deposited) crystalline fraction is in the range 0.01 to 25%. The typical as-deposited microcrystallite size is between 150 to 300 .ANG., although microcrystallites as large as approximately 1000 .ANG. are useful.
Step b) includes forming microcrystallites having a post-melt microcrystallite size and post-melt microcrystallite density. The post-melt size and density is the result of the annihilation of microcrystallites smaller than a microcrystallite critical size and the partial melting of microcrystallites having a pre-melt size larger than the critical size. Then, Step b) includes forming a post-melt microcrystalline film.
Because the melting process annihilates microcrystallites smaller than the critical size, the percentage of microcrystallites having a size less than the average, post-melt size, is smaller than the percentage of smaller than average sized pre-melt microcrystallites. Thus, the post-melt microcrystallites are more uniform in size. The more uniformly sized post-melt microcrystallites result better control of the post-melt microcrystallite density. The polycrystalline grains are more directly responsive to the post-melt crystalline fraction than most of the other process variables. A post-melt density of at least approximately 1.times.10.sup.8 microcrystallites per cm.sup.2 has been found to be effective, with approximately a 1 micron average separation between microcrystallites.
Preferably, Step b) includes uses an excimer laser crystallization (ELC) process, having a wavelength of approximately 308 nm, or less, to melt the amorphous matter and selectively melt the microcrystallites. The post-melt crystalline fraction is responsive to the ELC energy density as well as the pre-melt crystalline fraction.
In one aspect of the invention, Step a) includes depositing the microcrystalline film by a PECVD process using a SiH.sub.4 and H.sub.2 gas mixture. The deposition process includes using a power level of approximately 0.16783 W per cm.sup.2, at a temperature of approximately 320.degree. C., a total pressure of approximately 1.2 Torr, and a flow rate of SiH.sub.4 to H.sub.2 of approximately 100:1.
FIG. 1a shows deposition rate v. the percentage of crystalline fraction. The deposition rate is critical to the pre-melt crystalline fraction. When the microcrystalline film deposition rate is less than 2 .ANG. per second (.ANG./s), a crystalline fraction as high as 80% can be formed. Higher deposition rates can be used to form a crystalline fraction in the range between 0.01% and 50%. Preferably, the microcrystalline film is deposited with the PECVD method at a deposition rate of less than 10 .ANG./s and a deposition temperature of approximately 380 degrees C.
A liquid crystal display (LCD) is also provided comprising a transparent substrate and a TFT polycrystalline semiconductor film, overlying the transparent substrate, which has an electron mobility of greater than 150 cm.sup.2 /Vs, a threshold voltage less than 2 volts, a grain size larger than 0.5 microns, and grain size uniformity of less than 10%. The polycrystalline film is formed from the above-described method. Namely, depositing amorphous matter in deposition conditions which result in a microcrystalline film having a pre-melt crystalline fraction; annealing the microcrystalline film, with the annealing process selectively melting of microcrystallites to from a post-melt crystalline fraction; and, forming the polycrystalline TFT film with an electron mobility responsive to the post-melt crystalline fraction.